This invention relates in general to electrostatography and in particular to magnetically attractable ferrite carrier particles useful in the magnetic brush type development of electrostatic latent images.
Electrostatographic imaging processes and techniques have been extensively described in both the patent and other literature, for example, U.S. Pat. Nos. 2,221,776, 2,277,013, 2,297,691, 2,357,809, 2,551,582, 2,285,814, 2,833,648, 3,220,324, 3,220,831, 3,220,833, and many others. Generally, these processes have in common the steps of employing a normally insulating photoconductive element which is prepared to respond to imagewise exposure with electromagnetic radiation by forming a latent electrostatic charge image. The electrostatic latent image is then rendered visible by a development step in which the charged surface of the photoconductive element is brought into contact with a suitable developer mix.
One method for applying the developer mix is by the well-known magnetic brush process. Such a process can utilize apparatus of the type described, for example, in U.S. Pat. No. 2,874,063 and often comprises a nonmagnetic rotatably mounted cylinder having fixed magnetic means mounted inside. The cylinder is arranged to rotate so that part of the surface is immersed in or otherwise contacted with a supply of developer mix. The granular mass comprising the developer mix is magnetically attracted to the surface of the cylinder. As the developer mix comes within the influence of the field generated by the magnetic means within the cylinder, the particles thereof arrange themselves in bristle-like formations resembling a brush. The bristle formations of developer mix tend to conform to the lines of magnetic flux, standing erect in the vicinity of the poles and lying substantially flat when said mix is outside the environment of the magnetic poles. Within one revolution the continually rotating tube picks up developer mix from a supply source and returns part or all of this material to the supply. This mode of operation assures that fresh mix is always available to the copy sheet surface at its point of contact with the brush. In a typical rotational cycle, the roller performs the successive steps of developer-mix pickup, brush formation, brush contact with the photoconductive element, brush collapse and finally mix release.
In magnetic-brush development of electrostatic images, the developer is commonly a triboelectric mixture of fine toner powder comprised of dyed or pigmented thermoplastic resin with coarser carrier particles of a soft magnetic material such as "ground chemical iron" (iron filings), reduced oxide particles, or the like. The conductivity of the ferromagnetic carrier particles which form the "bristles" of a magnetic brush gives some advantage over other modes of development. The conductivity of the ferromagnetic fibers or bristles provides the effect of a development electrode having a very close spacing to the surface of the electrophotographic element being developed. By virtue of this development electrode effect, it is to some extent possible to develop part of the tones in pictures and solid blacks as well as line copy. This ability to obtain solid area development with magnetic brush developing makes this mode of developing advantageous where it is desired to copy materials other than simple line copy.
However, most currently available ferromagnetic carrier particles have an electrical resistance which is too high to produce good quality solid area development. The various commercial carrier particles generally lack adequate conductivity because of the presence of an insulating surface layer of iron oxide, grease or other contaminants. Efforts to remove such surface contaminants often result in particles which have an even higher electrical resistivity. For example, washing or solvent treatment of iron carrier particles in an effort to remove contaminants merely exposes the surface of the underlying iron to aerial oxidation. The new layer of oxide often has far greater resistivity than the original contaminants. Such an oxide coating can be removed; however, special after-treatment and precaution in storage and handling are required in order to avoid any further oxidation.
Ferrite materials are gaining ever increasing importance in the electronics industry and in the electrostatographic arts. Their use as low conductivity magnetic core materials and as carrier materials for photoconductive insulating materials is well known. Briefly, ferrites may be described in general as compounds of magnetic oxides containing iron as a major metallic component. Thus, compounds of ferric oxide, Fe.sub.2 O.sub.3, formed with basic metallic oxides having the general formula MFeO.sub.2 or MFe.sub.2 O.sub.4 where M represents a mono or divalent metal and the iron is in the oxidation state of +3 are ferrites. Ferrites are also refered to as ferrospinels since they have the same crystal structure of the mineral spinel MgAl.sub.2 O.sub.4. However, not all ferites are magnetic such as, for example, ZnFe.sub.2 O.sub.4 and CdFe.sub.2 O.sub.4. This lack of magnetic property is due to the configuration of the ferrite lattice structure. Further, some ferrites, such as magnetobarite, BaFe.sub.12 O.sub.19, which exhibit permanent magnetic properties are referred to as hard ferrites. A "hard" ferrite is difficult to magnetize and demagnetize and thus is the type of ferrite that is desirable in a permanent magnet. A "soft" ferrite has the opposite property; it is easily magnetized and demagnetized. The softer the ferrite material is, the better it is suited to various electrical devices in which magnetization must be reversed very often per unit of time. If one plots the characteristics of a hard ferrite and a soft ferrite on a graph in which the imposed magnetic field forms the horizontal axis and the total magnetization forms the vertical axis, one obtains a characteristic curve resembling a thick S known as a hysteresis loop. A hard ferrite has a wide hysteresis loop and a soft ferrite has a narrow one. Since each traversal of a loop represents energy lost, a narrow loop is desirable in devices in which magnetization must be reversed frequently.
The ferrite materials of main interest in the electrostatographic arts are the soft ferrites. The soft ferrites may further be characterized as being magnetic, polycrystalline, highly resistive ceramic materials exemplified by intimate mixtures of nickel, manganese, magnesium, zinc, iron, or other suitable metal oxides with iron oxide. Upon firing or sintering, the oxide mixtures assume a particular lattice structure which governs the magnetic and electrical properties of the resulting ferrite.
Electrostatograhic carrier surfaces and carrier particles are generally made from or coated with materials having appropriate triboelectric properties as well as certain other physical characteristics. However, the carrier substrate as well as the surface thereof should not be comprised of materials which are so brittle as to cause either flaking of the surface of particle breakup under the forces exerted on the carrier during recycle. The flaking thereof causes undesirable effects in that the relatively small flaked particles will eventually be transferred to the copy surface thereby interfering with the deposited toner and causing imperfections in the copy image. Furthermore, flaking of the carrier surface will cause the resultant carrier to have nonuniform triboelectric properties when the carrier is composed of a material different from the surface coating thereon. This results in undersirable nonuniform pickup of toner by the carrier and nonuniform deposition of toner on the image. In addition, when the carrier particle size is reduced, the removal of the resultant small particles from the photoconductive plate becomes increasingly difficult. Thus, the types of materials useful for making carrier or for coating carrier, although having the appropriate triboelectric properties, are limited because other physical properties which they possess may cause the undesirable results discussed above.
While ordinarily capable of producing good quality images, conventional developing materials suffer further serious deficiencies in certain areas. The developing materials must flow freely to facilitate accurate metering and even distribution during the development and developer recycling phases of the electrostatographic process. Some developer materials, though possessing desirable properties, such as proper triboelectric characteristics, are unsuitable because they tend to cake, bridge, and agglomerate during handling and storage. Adherence of carrier particles to reusable electrostatographic imaging surfaces causes the formation of undesirable scratches on the surfaces during the image transfer and surface cleaning steps. The tendency of carrier particles to adhere to imaging surfaces is aggravated when the carrier surfaces are rough and irregular. The coatings of most carrier particles deteriorate rapidly when employed in continuous processes which require the recycling of carrier particles by bucket conveyors partially submerged in the developer supply such as disclosed in U.S. Pat. No. 3,099,943. Deterioration occurs when portions of the entire coating separate from the carrier core. The separation may be in the form of chips, flakes, or entire layers and is primarily caused by fragile, poorly adhering coating materials which fail upon impact and abrasive contact with machine parts and other carrier particles. Carriers having coatings which tend to chip and otherwise separate from the carrier core or substrate must be frequently replaced thereby increasing expense and loss of productive time. Print deletion and poor print quality occur when carriers having damaged coatings are not replaced. Fines and grit formed from carrier disintegration tend to drift and form undesirable and damaging deposits on critical machine parts. Many carrier coatings having high compressive and tensile strength either do not adhere well to the carrier core or do not possess the desired triboelectric characteristics. The triboelectric and flow characteristics of many carriers are adversely affected when relative humidity is high. For example, the triboelectric values of some carriers fluctuate with changes in relative humidity and are not desirable for employment in electrostatographic systems, particularly in automatic machines which require carriers having stable and predictable triboelectric values.
Thus, there is a continuing need for a better developer material for developing electrostatic latent images.